Plug fuse



Aug. s, 1933,

F. C. LA MAR PLUG FUSE Filed Aug. l, 1930 lllllllllll .1.:

Patented Aug. 8, 1933 UNITED STATE-s PATENT OFFICE PLUG FUSE Application August 1, 1930. Serial No. 472,267

1 Claim. (Cl. 20G-119) This invention relates to electric circuit breakers, particularly those in which a thermal effect of the current is employed to break a circuit, and more especially to devices ofthe type known com- 5 monly as plug fuses.

As commonly made and sold, the ordinary plug fuses of current carrying capacity from practically nothing to a capacity of in excess of 30 amperes are all of substantially and practically identical external dimensions so that a plug fuse of any current carrying capacity may be placed in any receptacle intended fora plug fuse. In consequence of this identity of size, it is possible to either unintentionally or intentionally overfuse a circuit, that is, place therein a 'thermal fuse of a carrying capacity greater thanvthe maximum allowable carrying capacity proper to safely protect the circuit and the electrical devices included therein.

The ordinary commercially sold plug fuses generally have the fuse element in a chamber which, for the purposes of visibility, is closed with a fragile mica window. This window is often damaged in handling so that an opening is left therein so that when the thermal element is volatilized a tongue of intensely hot ilame and gases protrudes from the fuse plug and coming in contact with combustible material, especially in the older installations, occasionally causes surrounding objects to take fire with all of the potentially disastrous results.

The plug fuse as ordinarily used is a rather expensive device because when the thermal element, which is a very minor part of the device,

as regards expense, is blown or fused, it is necessary -to discard the entire structure as it requires tools and skill not ordinarily possessed by the average user to replace the destroyed thermal element.

A principal object of this invention is to provide an electric circuit breaker of the type speciled which will be so constructed that after installation in a particular circuit and a destruction of the thermal element thereof, the restoration of the electrical continuity cannot result in ,over-fusing a circuit unless an entirely new device is obtained.

A further object of the invention is the provision of a fuse of-the type described which will provide a construction which willr enable the replacement of a thermal element without necessitating the replacement of the entire structure.

A further object of the invention is the production of an electric circuit breaker of the type specied of a construction such that the thermal element volatilized will be enclosed in a chamber within a chamber so that the walls of two different chambers must be broken before the hot flames and gases from a destroyed fuse may spurt upon nearby inflammable material.

A further object of the invention is the production of an electric circuit breaker of the type specied of a construction such that two separate and independent thermal elements are included therein in a series circuit, one of a lower carrying capacity than the other so that .the thermal element of lower carrying capacity will ordinarily be the only one destroyed upon the flow of an unusual current value, the construction to be such, that the ordinarily destroyed thermal element is replaceable without the use of too1s,or special appliances, and the construction ofthe replaceable fusible element being such, that of the ordinarily sold refills only those of proper carrying capacity may be used to replace a destroyed thermal element, but if by any chance the circuit gap ordinarily closed by the rell is closed by a device of excess current carrying capacity, nevertheless, the circuit would be amply protected by the second thermal element, so that in order to over-fuse a given circuit in which a proper plug fuse has -already been inserted it will be necessary to entirely remove the plug fuse and replace it by another plug fuse of different current carrying capacity.

Other objects and advantages will appear as the description of the particular physical embodiments selected to illustrate the invention progresses and the novel features thereof will be particularly pointed out in the appended claim.

In describing the invention in detail and the particular physical embodiments selected to illustrate the invention reference will be had to the accompanying drawing and the several views thereon, in which like parts are designated by like characters of reference throughout the several views, and in which:

Figure 1 is a longitudinal sectional view ofa plug fuse constructed in accordance with my `in vention: Fig. 2 is a part sectional and part elevational view of a fuse constructed in accordance with my invention; Fig. 3 is a top plan view with cover removed, of the device as shown by Fig. 1;, Fig. 4 is a View similar to Fig. 3 illustrating a modied construction of renewable thermal element; Figs. 5 and 6 are detailed views of clips and attached conductors for holding the renewable thermal element of my plug fuse; Fig. 7 is a separated View of a modied construction of renewable thermal element, clips and secondary thermal ele- 1 ment; Fig. 8 is a view of a modified form of renewable thermal element; Fig. 9 is a fragmentary part sectional and part elevational view of a modified form of thermal element container.

In Fig. 1 the body of the plug is designated 1. This body is made of an insulating material, such as a condensation product of phenol, or porcelain. In accordance with common `practice it would preferably be formed by a molding operation. The molding operation would produce a large central cavity 2; an orice 3; niches 43 and 4; orifice 5, and the helical threads 6 and 7.

To furnish a means Whereby'the body may be threaded into a socket and make electrical contact with one side of a circuit, the metallic thirnble 8, formed in any appropriate and suitable manner as by blanking in a die and then rolling the helical thread, is provided.

A headed screw 9 passes through the orifice 5 and threads into the electrical conducting member 10. This metallic screw 9 serves-to prevent the thimble 8 vfrom turning independently of the body 1 and also serves to connect electrically the thimble 8 with the electrical conductor 10.

A metallic screw 11 extendsthrough the orifice 3 into threaded engagement with the metallic conducting member 12. This metallic screw v11 has a rather large head and serves to connect the plug structure with one side of the electrical circuit of the socket into which the plug is placed and also serves to securely Ihold the electrical conducting member 12 in place.

Within the cavity 2 isplaced the small cup 13 which extends from the bottom of the cavity 2 nearly to the top thereof. This cup 13 is made of insulating material and preferably of the same kind of insulating material of which the body is formed. The cup ts quite snugly within the cavity 2, almost touching the walls all around, and in addition the space between the top of the cup 13 and the body 1 is sealed as at 14 by any suitable sort of sealing cement such as those having shellac as a basis, such as ordinary sealing wax.

'Ihe electrical conducting members or bus-bars a right angle is fashioned in the form of clips as 17 and 18, best shown in Figs. 5 and 6.

Between the clips 1'7 and 18 extends a thermal element 19, that is, an element which is melted, fused or volatilized by the passage therethrough of more than a given amount of current. This thermal element ispreferably enclosed within a container 20. The container 20 is made preferably of a refractory material, such as glass, and preferably is made of a refractory material which is transparent, such as glass. The ends of the thermal element 10 are electrically connected by anyl suitable means, such as soldering, to the metallic end caps 21 and 22 of the'container 20. M

AThe opening of the cavity 2 and the niches 43 and 4 are closed by the cap 23. This cap is formed in any usual or preferred manner as by blanking in a die and then rolling a helical thread therein. The top of the cap is in the form of an annulus providing the free space 24.

Between the cap 23 and the top of the body 1 is a protective and closing disc 25. This disc is preferably made of a refractory material, and preferably one which is transparent. Thin sheet mica of the transparent Variety is Well adapted as a material for forming this disc.

The plug fuse as shown by Fig. 1 embodies the construction and presents the conditions desired by applicant. The sleeve 8 is of such di. mensions and so constructed that it will t into the standard cut-outs or fuse plugs now commonly used throughout the country. The total over all dimensions from top to bottom, as viewed in Fig. 1, of the plug corresponds substantially to those of the ordinary and well known fuse plugs of commerce,-and therefore may well be used in any situation in which such well known fuse plugs may be used.

The thermal element 19 of the fuse plug, as shown in Fig. 1, is contained Within two chambers, one the refractory material chamber 20, the other the cavity 2, closed by the disc 25. If the thermal element 19 is fused or volatilized it will be necessary for both the Walls of the chamber 20 and the disc 25 to be disrupted lin order to allow intensely hot ames and gases to spurt out into contact with surrounding combustible material. If the disc 25 has been perforatedby handling-there still exists a uchamber wall, as a wall of the container 20, .between the thermal element 19 and surrounding objects.

If the thermal element 19 becomes disrupted by an abnormal flow of current then the cap 23 may be removed-and a new thermal element substituted. When this substitution is made the entire device comprising the container 20, thermal element 19 and caps 21 and 22 will be replaced. Such an assembly would be sold as a refill, replacement, or renewal. In order to prevent placing in the circuit a thermal element, vas 19, of larger carrying capacity than desired, the size of the metallic end pieces 21 and 22 for insertion in the clips 17 and 18 would be made such that all thermal elements rated at more than, say, 20 amperes capacity would have metallic ends 21 and 22 of a size too large to insert inr clips 17 115 and -18 of a fuse plug designed for carrying current less than 20 amperes.

Although the intentional or unintentional overfusing of a plug is largely guarded against by the making of the metallic ends 21 and 22 of dif- 120 ferent size thermal elements 19, nevertheless, this will not in all cases be sufficient, and to that end applicant further provides that the electrically conducting members 10 and`12 may also be made of fusible or thermal material, as shown in Fig. 7, in which the member corresponding to 10 is designated 26 and the member corresponding to 12 is designated 2'7. Both members 26 and27 are made of thermal material and are made of a capacity such that they will become disrupted if a ow of current larger than contemplated occurs in the circuit in which fuse plug has been placed. Although in Fig. 7 both 26 and 27 are formed of a thermal material, nevertheless, it is to be understood, of course, that one leg only of the circuit may be composed of a thermal element as it would be sufficiently protected thereby. If the conductors 16 and 12 are made of a thermal clement, as shown in Fig. l, then if the thermal element 19 were disrupted, 140 and a new thermal element 19' placed in the fuse plug, and this new thermal element 19 were of a carrying capacity greater than the carrying capacity of the thermal element 27 and 28 then, upon abnormal flow of current, the thermal element 26 or 27 would be disrupted upon the` ow of that current greater than the carrying ca pacity of the thermal element 27 and 28-regard less of the factv that the thermal element 19 might remain intact. In such a case it would be 150 necessary to obtain an entire new fuse plug, as the ordinary user would be unable or disinclined to attempt the re-fusing of the destroyed plug. The fact that the thermal element buried within the body of the fuse plug had been disrupted would, however, warn the user that he had attempted to over-fuse the circuit, and would so give him such notice as would warrant a cautious man in proceeding properly. Y

In Fig. 7, another form 28 of a container for the thermal element 19 comparable with the container 20 is shown. This container is made by folding a refractory material about the thermal element 19. Such refractory elementfor form--v ing the container 28 might well be thin sheet mica.

With the form oi* container shown Aat 28 a modified form of end member is used. In Fig. v'l the end members 29 and 30 are at sided and may well be formed of bent up sheet material, and so constructed that the ends as 31 `and `32 will clamp the'sides of the container 28. The thermal element 19 would be connected to the end members 29 and 30 in any usual and preferred manner as by soldering, which would be applicable as the members 29 and 30 are made of metal.

Using the form of container as shown in Fig. 7, the clips holding the ends of the container must be suitably modied so that clips, as 33 and 34, as shown in Fig. 7, would be used. These clips would have the sides close together so as to yreceive ends such as 29 and 30 but would be inappropriate to receive a circular end such as 21 and 22.

In Fig. 8 a further modication of the container is shown, in that', the container 35 is not provided with end pieces as 21 and 22 or bent up (dat metal pieces as 29 and 30, but the ends as 36 and 37 are covered with a powdered metal and the ends 38 and 39 of the thermal element 19 are bent around into contact with this powdered metal and the powdered metal and the ends 38 and 39 and the container 35 are all fastened together by the application of a suitable cementing solution thereto, such as, amylacetate, commonly known as banana liquor.

' Although in the device as shown by Fig. 1, the portions 40 and 41 of the body 1 prevent the metallic cover 23 from coming in contact either with members 21 and 22 or members 17 and 18, nevertheless, I may, as an alternative, construct the top in the manner as shown by Fig. 9, in which the member 23 has a lining 42 of insulating material between which 'and the member 23 is positioned the disc 25. r

Although I have particularly described particular physical embodiments of my invention and explained the construction and principle of operation thereof, nevertheless, I desire to have it understood that the particular physical embodiments 4selected are illustrative only, but do not exhaust the possible physical embodiments of the idea of means underlying my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

In a plug fuse, in combination: a body formed with a cavity; means on the outside of .the body for making connection to each side of an electric circuit; means extending into the cavity of the body making connection to each side of the electric circuit; two separated clip-like members Within the cavity electrically connected to said last named means with atleast one electrically connected by means of a thermal element; means within the cavity substantially covering the thermal element and a unitary thermal device extending between saidy clips, said unitary thermal device includng a thermal element enclosed in a chamber havingI electrical conducting ends between which the thermal element extends and means closing the cavity including a transparent member. FRANK C. LA MAR. 

